LCD displays offer a compact, lightweight alternative to CRT monitors. In spite of their advantages, however, LCD displays are limited in brightness, or, more properly, in luminance, particularly when viewed from an off-axis angle, where the optical axis is generally normal to the LCD display surface. When viewed straight-on, along the optical axis, an LCD display may have sufficient luminance for most laptop computer applications. However, as the angle of the viewer increases with respect to the optical axis, luminance diminishes quickly.
The transmissive LCD used in conventional laptop computer displays is a type of backlit display, having a light providing surface positioned behind the LCD for directing light outwards, towards the LCD. The light-providing surface itself provides illumination that is essentially Lambertian, that is, having an essentially constant luminance from a broad range of angles. With the goal of increasing on-axis and near-axis luminance, a number of brightness enhancement films have been proposed for redirecting a portion of this light having Lambertian distribution. Among proposed solutions for brightness or luminance enhancement for use with LCD displays and with other types of backlit display types are the following:                U.S. Pat. No. 5,592,332 (Nishio et al.) discloses the use of two crossed lenticular lens surfaces for adjusting the angular range of light in an LCD display apparatus;        U.S. Pat. No. 5,611,611 (Ogino et al.) discloses a rear projection display using a combination of Fresnel and lenticular lens sheets for obtaining the desired light divergence and luminance;        U.S. Pat. No. 6,111,696 (Allen et al.) discloses a brightness enhancement article for a display or lighting fixture. With the optical film disclosed in the '696 patent, the surface facing the illumination source is smooth; the opposite surface has a series of structures, such as triangular prisms, for redirecting the illumination angle. The film disclosed in the '696 patent refracts off-axis light to provide a degree of correction for directing light at narrower angles. However, this film design works best for redirecting off-axis light; incident light that is normal to the film surface may be reflected back toward the source, rather than transmitted;        U.S. Pat. No. 5,629,784 (Abileah et al.) discloses various embodiments in which a prism sheet is employed for enhancing brightness, contrast ratio, and color uniformity of an LCD display of the reflective type. In an embodiment disclosed in the '784 patent, the brightness enhancement film similar to that of the '696 patent is arranged with its structured surface facing the source of reflected light for providing improved luminance as well as reduced ambient light effects. Because this component is used with a reflective imaging device, the prism sheet of the '784 disclosure is placed between the viewer and the LCD surface, rather than in the position used for transmissive LCD systems (that is, between the light source and the LCD);        U.S. Patent Application Publication No. 2001/0053075 (Parker et al.) discloses various types of surface structures used in light redirection films for LCD displays, including prisms and other structures;        U.S. Pat. No. 5,887,964 (Higuchi et al.) discloses a transparent prism sheet having extended prism elements along each surface for improved back-light propagation and luminance in an LCD display. As is noted with respect to the '696 patent mentioned above, much of the on-axis light is reflected rather than transmitted with this arrangement. Relative to the light source, the orientation of the prism sheet in the '964 disclosure is reversed from that used in the '696 disclosure. It must be emphasized that the arrangement shown in the '964 disclosure is usable only for small, hand-held displays and does not use a Lambertian light source;        U.S. Pat. No. 6,356,391 (Gardiner et al.) discloses a pair of optical turning films for redirecting light in an LCD display, using an array of prisms, where the prisms can have different dimensions;        U.S. Pat. No. 6,280,063 (Fong et al.) discloses a brightness enhancement film with prism elements on one side of the film having blunted or rounded peaks;        U.S. Pat. No. 6,277,471 (Tang) discloses a brightness enhancement film having a plurality of generally triangular prism elements having curved facets;        U.S. Pat. No. 5,917,664 (O'Neill et al.) discloses a brightness enhancement film having “soft” cutoff angles in comparison with conventional film types, thereby mitigating the luminance change as viewing angle increases; and        U.S. Pat. No. 5,839,823 (Hou et al.) and U.S. Pat. No. 5,396,350 (Beeson et al.) disclose back-coupled illumination systems with light recycling features, including various prismatic structures such as trapezoidal prisms mounted against a transparent base wall. Directed to light redirection in illumination apparatus where heat may be a problem, the solutions described in the Hou '823 and Beeson '350 disclosures employ non-Lambertian light sources with reflectors and provide an output that is not highly uniform.        
FIG. 1 shows one type of prior art solution, a brightness enhancement article 10 for enhancing light provided from a light source 18. Brightness enhancement article 10 has a smooth side 12 facing towards a light providing surface 14, which contains a reflective surface 19, and rows of prismatic structures 16 facing an LCD component 20. This arrangement, as described in U.S. Pat. Nos. 6,111,696 and 5,629,784 (both listed above), and in U.S. Pat. No. 5,944,405 (Takeuchi et al.), generally works well, improving the on-axis luminance by refraction of off-axis light rays and directing this light closer to the normal optical axis. As FIG. 1 shows, off-axis rays R1 are refracted toward normal. It is instructive to note, however, that, due to total internal reflection (TIR), near-axis light ray R3 can be refracted away from normal at a more extreme angle. In addition, on-axis light ray R4 can actually be reflected back toward light-providing surface 14 for diffusion and reflection from reflective surface 19 rather than directed toward LCD component 20. This refraction of near-axis light and reflection of at least a portion of on-axis light back into light providing surface 14 acts to adjust illumination luminance with respect to viewing angle, as is described subsequently. By the action of light-providing surface 14 and reflective surface 19, a portion of the light that is reflected back from brightness enhancement article 10 is eventually diffused and again directed outward toward the LCD component at a generally normal angle.
The purpose of brightness enhancement article 10, then, is to redirect the light that is provided over a large angular range from light providing surface 14, so that the output light it provides to LCD component 20 is more narrowly directed toward normal. By doing this, brightness enhancement article 10 helps to improve display luminance not only when viewed straight-on, at a normal to the display surface, but also when viewed from oblique angles.
As the viewer angle from normal increases, the perceived luminance can diminish significantly beyond a threshold angle. The graph of FIG. 2 shows a luminance curve 26 that depicts the characteristic relationship of luminance to viewer angle when using the prior art brightness enhancement article 10. As expected, luminance peaks at the normal and decreases toward a threshold cutoff angle θcutoff each side of normal. A slight increase occurs after angle θcutoff; however, this effect is wasted light, not readily perceptible to the viewer due to characteristics of the LCD display itself.
With reference to luminance curve 26 in FIG. 2, there are a number of characteristics of particular interest for brightness enhancement components. One characteristic is the overall shape of the curve. The luminance over a range of viewing angles is proportional to the area under the curve for those angles. Typically, the peak luminance values occur at angles near normal, as would be expected. In order to obtain an improved range of view angles, a brightness enhancement article redistributes light, changing the shape of its respective luminance curve 26 accordingly. Another characteristic of interest relates to cutoff angles θcutoff. At angles beyond θcutoff, luminance will be significantly diminished. Light provided at angles beyond θcutoff is essentially wasted. Thus, it can be seen that there would be advantages to design techniques that allow some measure of control over peak luminance levels, θcutoff, and the overall shape of luminance curve 26. With the characteristic behavior of FIG. 2 in mind, the disclosure of U.S. Pat. No. 5,917,664 describes a brightness enhancement article that provides a “softer” cutoff characteristic, using prism structures of varying dimensions. The method of the '664 patent uses a complex arrangement of different surface prism structures to changes the shape of the brightness response curve accordingly, so that greater luminance is available at off-axis angles.
While the approach of the '664 disclosure provides some improvement of off-axis luminance, there are additional considerations that suggest the need for further modification of the brightness response curve for achieving improved off-axis luminance. Referring to FIG. 3, there are shown two light rays directed through LCD component 20: ray R5 at normal incidence N and ray R6 at an oblique angle Q. It has been observed that even though the light being provided along both rays R5 and R6 may have equal intensity at the source, the perceived brightness through LCD component 20 is diminished at oblique angle Q, due to characteristics of LCD structures. As a comparative range of values, for example, where light from ray R5, at normal incidence N to the surface of LCD component 20, has a normalized intensity of 1.0, light from ray R6 at oblique angle Q of 20 degrees from normal can have a relative normalized intensity of about 0.8. In effect, this LCD characteristic acts to at least partially offset increased light intensity provided by smoothing the brightness response curve. Thus, even when light can be provided over a broadened range of angles, LCD characteristics themselves constrain the luminance levels available at oblique viewing angles.
While conventional approaches, such as those noted in the prior art disclosures mentioned hereinabove, provide some measure of brightness enhancement, these approaches have some shortcomings. One salient drawback of prior art approaches relates to the difficulty of predicting light behavior and how it may be modified. That is, while an existing design may work, conventional methods do not appear to provide tools for sufficient control over factors such as the overall shape of luminance curves 26 and the value of cutoff angles θcutoff. Certainly, the effects of changes to shapes and dimensions of surface structures can be assessed empirically once a film is fabricated. However, trial-and-error design methods can be less than satisfactory for design of a brightness enhancement article that would serve well in a specific application and such methods can be costly for developing prototype films for this purpose.
As disclosed in the patents listed above, brightness enhancement articles have been proposed with various types of refractive surface structures, including arrangements employing a plurality of triangular prisms, both as matrices of separate prism structures and as elongated prism structures, with the apex of these prisms both facing toward and facing away from the light source. In a broader context, these and other types of surface structures have been proposed with LCDs for specialized purposes other than for luminance enhancement. For example, in an article entitled “P-29: Design of the Viewing-Angle-Controlling Film for LCD”, in SID 00 Digest, authors Li, Zhang, Zhang, and Zhang propose the use of a combination comprising both trapezoidal and ellipsoidal prism structures external to the LCD and facing away from the light source for controlling the viewing angle of the display. Authors Li et al. describe how manipulating dimensions of these prism structures enables optimization to suit applications which require an LC display at specific viewing angles within ±20 to ±90 degrees. Unlike brightness enhancement articles, however, the prismatic cell array of the Li et al. disclosure is designed to improve optical characteristics such as display color and contrast within the viewing angle range, rather than to redirect light for improved luminance.
In spite of the concerted effort that has been expended for improving display luminance, there is still room for improvement. LCD display equipment still requires multiple layers of films for enhancing brightness and improving contrast, adding complexity and bulk to display packaging. In contrast to prior art techniques that use complex structures to modify luminance curve shape and cutoff angles, simplified techniques for more accurate control of curve characteristics and cutoff angles would be advantageous. Thus, it can be seen that there is a need for a brightness enhancement article that is light-efficient and allows a measure of control of luminance characteristics including cutoff angle.